One common method of preparing expanded cellular products is to mix a molten polymeric resin with a physical blowing agent in a zone of high pressure and to extrude the mixture into a zone of lower pressure where the blowing agent expands. Foams of low density and having a wide range of uses can be economically produced at high extrusion rates using physical blowing agents.
However, foams produced with physical blowing agents generally do not have the fine cell structure, resiliency, and softness that can be achieved with chemical blowing agents, which tends to limit the market for extruded foams. High quality shoe soles, padded surfaces for exercise equipment, and protective padding can be made from foams prepared with chemical blowing agents.
Foams that are prepared with chemical blowing agents normally are prepared in a two-stage process that is more troublesome and expensive than extrusion foaming. Chemical blowing agents generally are not activated until after the extrusion so that the extruded resin can be cross linked. Chemical blowing agents release a high rate of gas, normally an inert gas, including nitrogen or carbon dioxide. In the absence of cross linking, a chemical blowing agent generally would produce a large number of open, coarse cells.
Foams can be cross linked by irradiation or by free-radical catalysts, including peroxides. In one method, a molten polymer resin is mixed with peroxide and a chemical blowing agent. The temperature is kept as low as possible to avoid premature cross linking. Premature cross linking makes the resin more difficult to extrude and increases the amount of heat generated. Extreme heat tends to produce foams that are unstable and are subject to collapse. The resin is extruded and the extrudate is heated to initiate cross linking and to activate the chemical blowing agent to produce a foam.
Cheng-Shiang U.S. Pat. No. 4,738,810 describes a method of preparing a foam with a chemical blowing agent. The foam is prepared from linear low density polyethylene. Excessive cross linking is said to be precluded by premixing a chemical bowing agent, a cross linking agent, and other polymeric components before mixing with the linear low density resin.
Ionomer is said to be useful as one of the other polymeric components. Ionomers are copolymers having an ionizable comonomer. Ionomers are normally prepared by copolymerization of ethylene with small amounts of an unsaturated carboxylic acid, followed by ionization of the acid group to yield a metal salt. The ionized groups act as meltable cross links.
Ionomers have been used to improve the toughness and opacity to films, including the multilayer films that are used in vacuum skin packaging. Ionomers have also been used in resins for extrusion foaming under certain circumstances.
For example, Watanabe et al. U.S. Pat. No. 4,102,829 discloses low density extruded foams prepared from a mixture of from about 5 to 65% polyolefin and from about 35 to 95% ionomer. The foams are said to have a good balance of properties, including thermal resistance, and are indicated to be useful as an insulation covering on pipes for an air conditioner.
O'Brien et al. U.S. Pat. No. 4,091,136 discloses a synthetic cork-like material for use as a closure for liquid containers that is composed of an extruded fine-celled polyolefin foam containing an ionomer. The O'Brien et al. patent describes preparing a polyethylene foam rod with from 0.5 to 35 weight percent of DuPont Surlyn ionomer in the foamable resin mixture. The presence of ionomer is said to provide sufficient structural strength to the polyolefin foam so that it can be handled in conventional corking equipment.
Cylindrical product profiles that characterize insulation covers for air conditioning pipes and synthetic cork generally result in relatively low shear in the extrusion process, on the order of 10 sec.sup.-1. Shear generates heat, which reduces melt strength and can be problematic, particularly at higher levels, resulting in unstable foams that tend to collapse.
Extrusion foaming of sheet product profiles generally results in higher shear in the extrusion process of about 100 sec.sup.-1 or more. Shear can be several orders of magnitude greater for the production of sheet than for cylindrical product profiles.
High shear generation means that the heat generated by extrusion can be problematic. The "processing window" of suitable operating parameters of shear, melt temperature, and extrusion throughput for producing foam sheet products is relatively narrow compared to cylindrical product profiles. The process of extrusion foaming of sheet products normally will not tolerate cross linking in the resin, particularly at higher shear rates. Cross linking can render a resin unprocessable, particularly at high shear. Accordingly, there is not believed to have been any disclosure or suggestion to incorporate ionomer into polyolefin resins for extrusion as sheet or plank.
It would be desirable if polyolefin foam sheet products could be prepared with the economies of the extrusion foaming process that could be competitive with foams prepared from chemical blowing agents. However, polyolefins are relatively low modulus polymers that normally do not have the melt strength to form extruded foams of the fine cell structure and resiliency that is achieved with cross linking and expansion with chemical blowing agents.